1. Field of the Invention
This invention relates to the field of viscosity sensors, and particularly to viscosity sensors made from micromechanical (MEM) devices.
2. Description of the Related Art
Many mechanical systems require fluids for their operation. Extending the life of such systems requires that such fluids must be maintained, including the replacement of spent and degraded liquids. One method of assessing the health of such a fluid is to measure its viscosity.
Many devices have been developed to measure viscosity. One approach uses a vibrating quartz or piezoelectric element that measures the shift in a device's resonant frequencies or change in quality factor, Q, in response to applied vibrations; the frequency shift is a measurement of the damping value, which varies with viscosity. The measurement of damping value, however, is not a linear measurement, and thus will not be useful over a wide viscosity range. Further, this manner of measuring viscosity introduces complexities because both compressive and shear forces are applied to the sensing element; the contributions of both components to the net response can complicate data interpretation and limit operating range.
Microelectromechanical (MEM) devices—i.e., integrated devices which include at least one moveable element that moves relative to a stationary element—have also been employed to measure viscosity; MEM-based viscosity sensors are described, for example, in co-pending patent application Ser. Nos. 10/956,229 and 11/222,721 and assigned to the same assignee as the present application. In operation, the sensor is immersed in the fluid being assessed, the moveable element is commanded to move through the fluid, and the time it takes to respond is measured. The time response varies with the fluid's viscosity.
These devices use lateral motion in a fluid to generate the sort of nearly pure shear response required for accurate viscosity measurements. Movable and stationary interdigitated comb structures are moved laterally with respect to each other, with the resulting capacitance varying with the amount of overlap between the structures. However, due to the close proximity of the movable and stationary comb structures, this type of device may be particularly sensitive to particulate contamination.